LAUSR

The assimilation of inorganic nutrients by phytoplankton strongly depends on environmental conditions such as the availability of nitrogen (N) and temperature, especially warming. The acclimation or adaptation of different species to such changes remains poorly understood. Here, we used a multimethod approach to study the viability, and physiological and biochemical responses of the marine diatom Chaetoceros pseudocurvisetus to different temperatures (15, 25, and 30 °C) and different N: phosphorus (P) ratios (N-excess (N:P = 50:1), optimal (N:P = 16:1), and N-limiting (N:P = 2:1)). N limitation had a greater effect than high temperature on cell growth and reproduction, leading to a marked elongation of setae, decreased P assimilation, increased lipid accumulation and decreased protein synthesis. The elongation of setae observed under these conditions may serve to increase the surface area available for the uptake of inorganic and/or organic nitrogen. In contrast, high temperatures (30 °C) had a stronger effect than N deficiency on cell death, N assimilation, chlorophyll a accumulation, the cessation of setae formation and cell lipid remodelling. Significant changes in thylakoid lipids were observed in cells maintained at 30 °C, with increased levels of digalactosyldiacylglycerols (DGDG) and sulfoquinovosyldiacylglycerols (SQDG). These changes may be explained by the role of galactolipids in thylakoid membrane stabilization during heat stress.